Low temperature synthesis of CaZrO3 nanoceramics from ... · Low temperature synthesis of CaZrO 3...

Vol.48, No.1, June 2015, pp.17-25

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Low temperature synthesis of CaZrO3 nanoceramics from CaCl2–NaCl

molten eutectic salt

Rahman Fazli1*

and F. Golestani-Fard2

1. Young Researchers and Elite Club, Zanjan Branch, Islamic Azad University, Zanjan, Iran

2. School of Metallurgy and Materials Engineering, Iran University of Science and Technology, Tehran, Iran

Received: 13 Dec. 2014 ; Accepted: 6 May 2015

*Corresponding Author Email: [email protected], Tel: +98-937-2139453

Abstract

CaZrO3 nanoceramics were successfully synthesized at 700 C using the molten salt method, and the

effects of processing parameters, such as temperature, holding time, and amount of salt on the

crystallization of CaZrO3 were investigated. CaCl2, Na2CO3, and nano-ZrO2 were used as starting

materials. On heating, CaCl2–NaCl molten eutectic salt provided a liquid medium for the reaction of

CaCO3 and ZrO2 to form CaZrO3. The results demonstrated that CaZrO3 started to form at about 600C

and that, after the temperature was increased to 1,000C, the amounts of CaZrO3 in the resultant powders

increased with a concomitant decrease in CaCO3and ZrO2 contents. After washing with hot distilled

water, the samples heated for 3 h at 700C were single-phase CaZrO3 with 90–95 nm particle size.

Furthermore, the synthesized CaZrO3 particles retained the size and morphology of the ZrO2 powders

which indicated that a template mechanism dominated the formation of CaZrO3 by molten-salt method.

Keywords: calcium zirconate, molten salt method, nanomaterials, template growth.

1. Introduction

Calcium zirconate (CaZrO3) due to its valuable

properties, such as high melting point

(2,340C), high dielectric permittivity, and

low dissipation factor, is a ceramic material

that is currently being used in a wide range of

applications: multilayer ceramic capacitors,

solid electrolyte, crystalline host for

phosphorescence materials, moderate

temperature thermal barrier catalyst, and so on

[1–3]. There are several methods for the

synthesis of this material. CaZrO3 powders is

conventionally synthesized via a high

temperature (1500C) solid state reaction of

powdered calcium oxide (CaO) (or calcium

carbonate (CaCO3)) and zirconia (ZrO2)

(conventional mixed oxide synthesis (CMOS)).

As the reactions are generally controlled by

slow diffusion mechanisms, highly reactive

raw materials, high temperatures, and long

duration have to be used for the reactions to

Journal of Ultrafine Grained and Nanostructured Materials

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Fazli & Golestani- Fard / Vol.48, No.1, June 2015

18

achieve completion. The resultant product is

a hard mass, which often needs to be crushed

and ground to achieve the desired particle size

[4]. Other methods such as electrofusion [5],

wet chemical [6–8], combustion [9], and

mechanical alloying (MA) [10] have been

reported for synthesis of calcium zirconate.

Almost all the aforementioned methods are not

commodious, because their synthesis

temperatures are high in solid state and

electrofusion methods and thus need so much

thermal energy and time. Therefore, it is

necessary to follow methods that decrease

synthesis temperature and time. Besides the

above techniques, a low temperature

synthesis technique called molten salt

synthesis (MSS), is beginning to attract

interest. In this method, a salt is used as a

liquid medium, therefore, the reactions are

faster and synthesis is complete in significantly

lower temperature and time [4, 11, 12]. Zushu

Li et al.’s investigation is perhaps the most

important research on the synthesis of CaZrO3

via molten salt method that prepared CaZrO3

powder at 1,050C for 5h [4]. In this work,

CaZrO3was synthesized by heating CaCO3,

sodium carbonate (Na2CO3), sodium chloride

(NaCl), and nano-ZrO2 mixture, and the effects

of temperature, holding time, and salt to oxide

ratio on synthesis process were investigated.

Also, synthesis mechanism was analyzed.

2. Experimental Procedure

2.1. Dispersion of nano-ZrO2

Nano-ZrO2 (Neutrino, Germany, D50 = 60

nm, > 99% pure), hydrochloric acid (HCl),

and distilled water (pH = 7.2) were used for

the dispersion of nano-ZrO2. First, pH of

distilled water was decreased to 4 using HCl

by gently adding it to distilled water and then

measuring the pH using pH meter (Jenway,

model 3540). After regulating the pH of

distilled water, agglomerated nano-ZrO2

particles were added and stirred for 1h using

electrical stirrer. Then, this suspension was

placed for 2min in an ultrasonic probe and

then stirred for 1h again. The process of

stirring and placing in ultrasonic probe were

alternatively repeated several times to obtain

extremely dispersed particles. Then, a little

amount of the stirring suspension was poured

on a hot glass lamella using a syringe.

Thereafter, water from the suspension was

rapidly vaporized, separated and dispersed

particles of nano-ZrO2 remained in the

container. The particle size of dispersed

nano-ZrO2 was determined via dynamic light

scattering (DLS, Malvern, ZEN 3600),

scanning electron microscopy (SEM, Tescan

Vega II), and transition electron microscopy

(TEM, CM 200, Philips).

2.2. Preparation of powder mixture

Na2CO3 (Merck, Germany, D50 = 1mm, 99.5%

pure), calcium chloride (CaCl2) (Merck,

Germany, D50 = 4 mm, 99.5% pure), and nano-

ZrO2 (Neutrino, Germany, D50 = 60 nm, > 99%

pure) were used as starting materials. First,

stoichiometric compositions of Na2CO3 and

CaCl2 were completely mixed and the obtained

dual mixture was heated at 150C for 12h to

dry. Then, this salt was added to completely

dispersed nano-ZrO2 solution and the final

suspension was stirred for 1 h to homogenize

extremely. The mixture was fully dried at

120C for 12h. Molar ratio of final mixture was

ZrO2:Na2CO3:CaCl2 = 1:1:1.4. Agglomerations

of obtained powder which was completely

homogenous were broken using an agate

mortar and then sifted to pass through a 325

mesh screen (45 µm). Finally, the mixture (20

g) was placed in an alumina crucible covered

with an alumina lid, heated to 600; 650; 700;

800; 900; and 1,000C and held for 1, 3, and

5h. For investigating the effect of salt to oxide

ratio on the synthesis process, the samples were

heated at optimum temperature with 1:1, 2:1,

3:1, and 4:1 salt to oxide ratios. The heating

and cooling rates were 3 C/min and 5 C/min,

respectively. After cooling to room

temperature, the solidified mass was washed

and filtered in hot distilled water five times to

remove the salts. Then, the obtained powder

was dried at 120 C for 4 h. The phase

formation, morphology, and elemental analysis

of the synthesized powders were characterized

via X-ray diffraction (XRD, Philips pw3710),

SEM (TescanVega II), and X-ray fluorescence

(XRF, Philips 2404), respectively.

3. Results and discussion

3.1. Dispersed nano-ZrO2

SEM micrograph of agglomerated nano-ZrO2

particles has been shown in Figure 1. It is


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obviously observed that particle size of

zirconia was in the range of 200–250 nm.

This issue can be attributed to adherence and

thereupon agglomeration of nano-ZrO2

particles.

Fig. 1. SEM micrograph of agglomerated nano-ZrO2

particles

SEM micrograph of dispersed nano-ZrO2

particles has been shown in Figure 2. As

seen, particle size of nano-ZrO2 was in the

range of 60–80 nm. This object was

confirmed by TEM micrograph of these

particles shown in Figure 3.

Fig. 2. SEM micrograph of dispersed nano-ZrO2

particles

Fig. 3. TEM micrograph of dispersed nano-ZrO2

particles

Figure 4 shows particle size distribution

curve of nano-ZrO2 suspension measured via

DLS method. It is seen that distribution of

particle size was centralized in the range of

60–70 nm which means that almost all

agglomerations were broken and nano-ZrO2

particles were successfully dispersed.

Fig. 4. Particle size distributing curve of nano-ZrO2

suspension

3.2. Thermogravimetric analysis (TGA) of

synthesis process

DTA/TG analysis was performed to determine

the proper reaction temperature range as well

as the reaction order in the molten salt method.

Figure 5 indicates DTA/TG curve of Na2CO3,


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CaCl2, and nano-ZrO2. The DTA curve

exhibits an endothermic peak (peak a), which

is associated with a slow weight loss (10%) in

the TG curve at 100C. This weight loss is

attributed to dehydration of the precursors.

The small endothermic peak at approximately

150C (peak b) is related to the reaction

between Na2CO3 and CaCl2. The big

endothermic peak at about 500C (peak c) is

attributed to melting of CaCl2–NaCl eutectic

salt. The exothermic peak at approximately

600C (peak d) which is associated with a

slow weight loss in the TG curve is related to

formation of CaZrO3. The small exothermic

peak at 700C (peak e) is interpreted as the

crystallization of the CaZrO3 phase.

Fig. 5. DTA/TG curve of CaCl2, Na2CO3, and nano-

ZrO2 mixture

3.3. Effect of temperature

Figure 6 shows XRD patterns of the samples

heated for 3h at different temperatures. As

obviously observed, optimum temperature for

these samples was 700C. At this

temperature, the samples were single-phase

CaZrO3 and CaCO3 and ZrO2 peaks were not

observed. In other words, ZrO2 and CaCO3 were

completely transformed to CaZrO3. At

temperatures above 700C, the samples were

likewise single-phase CaZrO3 and just their

crystallinity was increased. This object was

confirmed by means of increase in peak’s

intensity. At 1,000C, the peak’s intensity

were insignificantly decreased and partly

become wider that can be attributed to acceding

decomposition temperature of CaZrO3. Thus,

increase in temperature was a very effective

factor for completion of synthesis process.

Fig. 6. XRD patterns of the samples (water washed)

heated for 3 h at different temperatures

Energy dispersive X-ray spectroscopy

(EDS) micrograph of the samples heated at

700C for 3h shown in Figure 7 confirmed

that optimum temperature for synthesized

samples was700C. As seen, almost only [Ca],

[Zr], and [O] elements were observed and

other elements were eliminated.

Fig. 7. EDS micrograph of the samples heated at 700C for 3h


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3.4. Effect of holding time

Figure 8 provides XRD patterns of the

samples heated for different holding times at

700C. As observed, optimum holding time

for these samples was 3 h. In this holding

time, synthesis process was complete and the

samples were single-phase CaZrO3. Also,

CaCO3 and ZrO2 peaks were not observed. In

1h holding time, CaCO3 and ZrO2 peaks were

seen, in addition to CaZrO3 peaks. In other

words, synthesis process was not completed.

In 5 h holding time, the samples were

likewise single-phase CaZrO3 and just

intensity of their peaks was insignificantly

increased compared with 3h holding time,

which means that their crystallinity was

venially increased. Thus, increase in holding

time, before optimum holding time, was an

effective factor for completion of synthesis

process and after this holding time did not

have a significant effect.


heated at 700C with different holding times

3.5. Effect of salt amount

XRD patterns of the samples heated with

different salt to oxide ratios at 700C have

been shown in Figure 9. It resulted in an

optimum salt to oxide ratio of 2:1, as the

synthesis process was completed in this

ratio. In salt to oxide ratio of 1:1, the

samples still did not become single-phase

CaZrO3, and it was necessary to increase the

salt to oxide ratio for completion of the

synthesis. With increase in the salt to oxide

ratio of 2:1 to higher values, the samples

remained single-phase and during more

increase in salt to oxide ratio, more increase in

peaks intensity and thereupon more increase in

crystallinity property were seen. Thus,

increase in salt to oxide ratio was a very

effective factor for the completion of

synthesis process.


heated at 700 C for 3 h with different salt to oxide

ratios

3.6. Particle size

Figure 10 shows SEM micrographs of the

samples synthesized at different temperatures.

As seen, the particle size of CaZrO3

synthesized at 700, 800, and 900C was in the

range of 90–95 nm. Whereas, the particle size

of CaZrO3 synthesized at 1,000C was in the

range of 180–200 nm which can be attributed

to grain growth phenomenon started at above

1,000C.

Particle size distribution curve of CaZrO3

particles synthesized at 700C for 3h

measured via DLS method has been shown in

Figure 11. As seen, distribution of particle

size was centralized in the range of 90–95

nm.


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Fig. 10. SEM micrographs of the samples synthesized for 3 h at (a) 700C; (b) 800C; (c) 900C; (d) 1,000C

Fig. 11. Particle size distribution curve of CaZrO3 particles synthesized at 700C for 3h


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3.7. CaZrO3 synthesis mechanism

NaCl, CaCl2, or NaCl–CaCl2 salts could be

used as molten salt media. According to

Figure 12 [13], as melting point of eutectic salt

(504C) was lower than both singular CaCl2

(771C) and NaCl (801C) salts, NaCl–

Na2CO3 eutectic salt was more suitable.

Fig. 12. CaCl2 –NaCl phase diagram [14]

On heating the mixture, the first reaction

was between Na2CO3 and CaCl2 (Reaction 1).

2 2 3 3

0

2

118123 198

CaCl Na CO CaCO NaCl

G T

(1)

This is consistent with the thermodynamic

prediction that reaction 1 could occur at as low

as room temperature, because the ∆G0 is

negative in the whole temperature range

between 25 and 600C [14]. Once Reaction

(1) was completed, Na2CO3 would disappear,

and the salt assembly would become one that

is essentially composed of NaCl and excess

CaCl2. The molar ratio between NaCl and the

excess of CaCl2 is 2:0.4, and the NaCl–CaCl2

phase diagram indicates that salt with such a

composition will start to melt at 504C

(eutectic temperature) in the areas where the

eutectic exists and become completely liquid

at about 650C (liquidus temperature).

This molten NaCl–CaCl2 salt provided a

reaction medium for the CaZrO3 synthesis

and also acted as a very proper catalyzer. Thus,

Reaction 2 started at about 600C that was

very lower than its thermodynamic prediction

(670C) [14]. With increase in the

temperature to 600C, CaCO3 reacted with

ZrO2 and some CaZrO3 was formed. With

increase in the temperature to 650C, NaCl–

CaCl2 salt completely melted, and reaction 2

became very more complete and rapid. Due

to very high reactivity of nano size ZrO2,

synthesis of CaZrO3 was completed at 700C.

3 2 3 2 0

193400 289

CaCO ZrO CaZrO CO

G T

(2)

To understand the reaction mechanisms,

the whole synthesis process of CaZrO3

discussed above has schematically been

illustrated in Figure 13.

Two main mechanisms, “template-growth”

and “dissolution-precipitation,” were involved in

MSS. Solubility of reactants in the molten salt

plays an important role in MSS. This not only

affects the reaction rate but also the

morphologies of the synthesized particles. If

both of the reactants are soluble in the molten

salt, then the product phase will be readily

synthesized via precipitation from the salt

containing the dissolved reactants

(dissolution–precipitation mechanism). In this

case the morphologies of the product grains

will generally be different from those of the


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reactants. However, if one of the reactants is

much more soluble than another, the more

soluble reactant will dissolve into the salt firstly

and then diffuse onto the surfaces of the less

soluble reactant and react in situ to form the

product phase. In this case, the morphology of

the synthesized grain will retain the less soluble

reactant (template-growth mechanism) [15, 16].

Fig. 13. Schematic diagram illustrating the synthesis of CaZrO3 powder by

heating nano ZrO2, CaCl2, and Na2CO3

According to some studies [17, 18],

CaCO3 is soluble in a chloride molten salt. Its

solubility in a NaCl-based salt at 700–1,000C

are on the order of 10−3

(molar fraction),

which is thousand times higher than that of

ZrO2 (on the order of 10−6

). Therefore, during

the MSS process, CaCO3would be dissolved

more in the NaCl–CaCl2 molten salt and react

with ZrO2 templates to form in situ CaZrO3.

This explains the similarity between the grain

shapes of the synthesized CaZrO3 and original

ZrO2 powder. The morphology and particle size

of the synthesized CaZrO3 grains was similar to

ZrO2 grains which mean that the “template-

growth” mechanism played a dominant role

in the low-temperature molten salt synthesis

of CaZrO3 nanoparticles (Fig. 14).

The chemical composition of product

powders determined via XRF analysis has

been shown in Table 1. As seen, after

washing, only minor contents of salt and other

contaminations remained in the synthesized

CaZrO3 powders. The main objective of this

table is to illustrate the feasibility of the

molten salt method for the synthesis of very

pure ceramic powders.

Table 1. Chemical composition of synthesized CaZrO3 in different reaction conditions

Weight Percent Reaction Condition

Al C Cl Si O Zr Ca Salt to oxide ratio Time (h) Temperature (oC)

0.01 0.03 0.08 0.14 59.92 19.89 19.93 2:1 3 600

0.01 0.02 0.08 0.12 59.93 19.90 19.94 2:1 3 650

0.02 0.01 0.07 0.13 59.93 19.91 19.93 2:1 3 700

0.01 0.01 0.08 0.11 59.92 19.92 19.95 2:1 3 800

0.02 0.02 0.07 0.12 59.91 19.92 19.94 2:1 3 900

0.02 0.03 0.06 0.12 59.91 19.91 19.95 2:1 3 1000

0.01 0.03 0.08 0.11 59.92 19.91 19.94 2:1 1 700

0.01 0.02 0.07 0.11 59.90 19.93 19.96 2:1 5 700

0.01 0.01 0.08 0.12 59.91 19.92 19.95 1:1 3 700

0.01 0.01 0.07 0.12 59.92 19.91 19.96 3:1 3 700

0.01 0.01 0.07 0.13 59.92 19.90 19.96 4:1 3 700


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4. Conclusions 1. Nano-size calcium zirconate powders

were synthesized via molten salt method.

Na2CO3, CaCl2, and nano- ZrO2 were used as

starting materials.

2. Optimum temperature for samples was

700C that was significantly lower than that

required by solid state method. Increase in

temperature was a very effective factor for

completion of synthesis process.

3. Optimum holding time for samples was

3h.Increase in holding time significantly did

not affect synthesis process.

4. Optimum salt to oxide ratio for samples

was 2:1. Increase in salt to oxide ratio resulted

in increase in the crystallinity property. Also,

increase in salt to oxide ratio was an effective

factor for completion of synthesis process.

5. Particle size of synthesized CaZrO3 was

90–95 nm. At above 1,000C, grain growth

phenomenon caused increase of particle size.

6. Similarity of morphology and particle

size of synthesized CaZrO3 to ZrO2 grains

showed that “template–growth” was dominant

mechanism in synthesis process.

7. Very minor contaminations components

remained in the synthesized powders indicated

that very pure CaZrO3nanoceramicswas

formed from CaCl2–NaCl molten eutectic salt.

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